Electric vehicles or hybrid vehicles, which have a motor as a drive power source for a vehicle, are proposed recently from the standpoint of social demands of low fuel consumption and low exhaust emission. JP 2009-27848A discloses one exemplary electric power supply control system for a vehicle. This system is provided with a relay (contactor), which has a positive-side switch and a negative-side switch, between a motor and a fuel cell stack. The motor is a drive power source for a vehicle and the fuel cell stack is an electric power source for the motor. In this power supply control system, the following control is proposed for improving durability of the relay. When the relay is switched over to an off-state, both of the positive-side switch and the negative-side switch are turned off at different timings with a certain delay therebetween. The order of turning off the positive-side switch and the negative-side switch is reversed at every predetermined number of switching operations. Thus arc discharges are generated when the switches are turned off evenly at the positive switch and the negative switch.
However, the positive-side switch and the negative-side switch are turned off with only the certain shift in timing when the relay is turned off. The number of operations of each switch is not decreased, because both of the positive-side switch and the negative-side switch are turned off. It is therefore not possible to prolong the period required for the number of switching operations reaches an allowable upper limit and to improve durability life of the relay.